The best option would be:
I have joined a site which is focused on learning English.
Reason- You have joined the site and you are still in the site and you did not leave it. So the result of your last action (joining the site) still persists.
Why is Your No 1 option not Fully appropriate-
I joined a site does not imply that you are still in the site, which is not true. You are in it and you did not leave the site. You may still be in and it maybe that you have left the site lately which is not written.
Why Your No 2 Is Fully inappropriate-
which was focused on learning English. implies that the site was at one point, focused on learning English, but it is not focused on learning English now, which is wrong.
If you are learning English, I would suggest that you not follow any hard and fast rule for joining sentences. When you join sentences, just make sure the resultant sentence keeps the meaning of both the original sentences.